Advanced composite structures are generally composed of multiple layers of high strength materials in fabric or tape form, impregnated with polymer resins. Presently, most such structures are formed or "laid up" on female molds which are formed in the desired shape of the finished product. Many of these structures, in order to provide additional strength or to provide additional thickness to the part are made with a semi-rigid core material sandwiched between layers of composite material. Where it is required to attach fittings to a composite part or openings are required in the surface of the part, the structural core material is removed in a certain area and the edges of the opening are chamfered before the core is laid in the mold. Typically, the outer skin of the structure is laid into the tool and a layer of adhesive is added to the inside of this skin. Next, the core, with its cut-out opening is placed on top of the outer skin. After adding another layer of adhesive, the fabric or tape which will be the inner skin of the finished structure is placed on top of the structural core.
The assembly is next placed in a pliable vacuum bag, typically Nylon, and the bag is sealed to the mold around the part. A slight vacuum is drawn on the inside of the bag in order to compress the structure and hold it in place. The mold with the sealed structure is then placed in a pressure chamber such as an autoclave, where the pressure is increased to further compress the part to form the shape of the mold and heat is applied according to a predetermined schedule to cure the part. The pressure of the autoclave causes the inner skin to conform to the chamfered opening in the core and the desired opening shape results.
Today, instead of laying sheets of tapes or fabric in female molds as described above, new methods and automated machinery have been developed which allow the material, in the form of thin strips or "tows", to be applied directly to a male mold or to a tool. This modern method allows large parts to be fabricated with considerably less labor required to fabricate the part. However, when this fiber placement method is used, the creation of openings in the structural core becomes more difficult. When using a male mold or tool the chamfered core openings are inverted and a tool core must be placed on the tool surface to form an opening in the finished product. Typically, the tool cores are fastened to the tool surface and the structural core is cut out in the same shape and is intended to be located directly above the tool core such that when the structure is cured, the desired opening will result.
The problem arises because the structural core openings are cut by a technician using a template. First, it is difficult to carve the openings to exact dimensions and, in addition, the structural core is not rigid and tends to deform when it is handled. Thus it is desirable to find a means of installing tool cores on a male tool such that the desired openings are formed in the correct place and are of the proper size when the part is formed by an automated fiber placement method.